Flexible, rotatable, drive cables are used in a variety of machines and apparatuses as a means to transmit mechanical rotary motion. These drive cables are commonly employed in machinery, such as concrete vibrators, automobiles, tractors, and lawn and garden equipment, including lawn edgers and weed trimmers. Flexible drive shafts require an insulative lining that centers the cable within its outer support tubing. The lining is usually a sheath of plastic or a composite of metal and plastic.
In a weed trimmer, as a typical example, a flexible shaft is centered within an aluminum tube placed between the gasoline engine or electric motor, and the nylon line cutting head. The flexible shaft is centered within the external aluminum tube by a centering assembly. A typical centering assembly is illustrated in U.S. Pat. No. 5,364,307, issued to John Shaulis on Nov. 15, 1994, for COAXIAL DRIVE CABLE CENTERING APPARATUS. The centering assembly of this patented device features a plastic liner of 6/6 high-temperature nylon, over which a polypropylene star-shaped member is co-extruded. The assembly conforms to the inside diameter of the aluminum tube, as well as to its longitudinal bend configuration. The outer, star-shaped member centers the inner nylon liner, as well as the flexible drive shaft, with respect to the aluminum tube. It also aligns the shaft with respect to the engine and the nylon line cutting head.
One of the drawbacks of the aforementioned liner is the expense of having to extrude two separate sleeves. That is, the star-shaped, propylene member must be co-extruded over the inner 6/6 nylon tube.
Another disadvantage of this liner relates to the double layers, which add thickness, material, weight and manufacturing cost to the liner assembly.
A further drawback of this liner is a result of the vibration caused by the rotating shaft, which vibration can cause premature wear of the shaft and components attached thereto. High frequency vibration can also be harmful to the operator. Prolonged exposure to vibration has been known to cause carpal tunnel syndrome.
Due to the screw effect of the rotating core, the lubricating grease tends to travel or migrate along the smooth inner surface of the liner, resulting in non-uniform grease lubrication when the grease deposits at the bottom end of the edge trimmer.
The present invention features a single centering liner for a flexible shaft that is lower in cost to manufacture, is both lighter and thinner than the prior two-pieced liner, and prevents non-uniform lubrication distribution. It also reduces the transfer of vibration to the outer support housing and, ultimately, to the operator.
As aforementioned, the current invention comprises a single liner element. It is, therefore, an improvement over the prior liner, by reason of a reduction in weight, number of parts and manufacturing complexity.
The liner of this invention comprises a hollow tube having a plurality of flared sections that are spaced apart along its length. The internal bore of the liner provides bearing support for the rotating flexible shaft during its operation.
The flared sections of the tube serve several purposes. The spaced-apart, flared sections center and locate the liner within the outer aluminum tube that supports the flexible shaft and liner assembly. They also reduce vibration, harmonics, and resonance produced by the rotating drive shaft, by providing nodal interference along the length of the tube. The nodal interference results from the change in tube diameter at these flared sections. The increased diameter interrupts vibrational and resonant longitudinal propagation at these spaced-apart sections. These flared sections also reduce radially-propagated vibration and shock forces transmitted from the drive shaft, by separating the drive shaft from the liner at these nodal centers. The expansion of the tube at these nodal points cushions shock and vibration, which is interrupted by a substantial layer of air.
Prior to the insertion of a liner within the end product (e.g., the weed trimmer), the flexible drive shaft is lubricated with grease. During operation, the grease tends to travel along the rotating drive shaft, as aforementioned. The nodal, flared sections of this invention act as retainers of the grease and disrupt the outward migration of the grease along the shaft. The grease pools within the pockets created by the flared sections. During operation, friction from the rotating drive shaft heats and softens the grease pools. The grease then flows from these pockets, thus improving the distribution of the lubricant along the shaft.